Electrodynamic device



June 27, 1933. w GlLEs 1,915,358

ELECTRODYNAMIC DEVICE Original Filed April 11, 1930 2 Sheets-Sheet 1 I IIIIIIIIIIII iwx k\\\\ ull-mum" lNVENTOP LW GILES 5) why 6. M

June 21, 1933- L. w. GILES 1,91 ,3 8

BLECTRODYNAMIC DEVICE Original Filed April 11, 1930 2 Sheets-Sheet 2 A. W GlLE$ B) A TTOPNEY Patented June 2 11933 UNITED STATES- PATENT OFFICE LEONARD W. GILES, OF BROOKLYN, NEW YORK, ASSIGNOB TO BELL TELEPHONE LABO- BATOBIES, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK nmc'rnonynmc DEVICE Application filed April 11, 1930, Serial No. 448,491. Renewed November 11, 1932.

This invention relates to electrodynamic devices and particularly to such devices for use in transmitting and receiving sound waves.

/ attached to the magnetic structure so as to vibrate when variable currents pass through the movable coil and change the constant magnetic flux in the annular gap in which the coil is suspended. The diaphragm is controlled in its movement by providing a chamher under the diaphragm and utilizing the mass, elasticity and viscosity of the air to efficiently damp the mechanical motion of the diaphragm This chamber is provided by substantlally closing the annular gap, at the side of the pole pieces opposite the iaphragm and coil, with a damping plate which is supported wholly by the inner pole piece.

In one embodiment of the invention, the damping plate is maintained in roper relation' with'the pole piecesby a t in spacing member having extensions which bridge the gap between the pole pieces and insure a definite restricted path for the passage of air from the chamber. This arrangement insures close calculation of the area of the restricted slot through which the air from the damping chamber must pass, so that the proper acoustic. impedance of the air in the chamber when associated with the impedance of the diaphragm produces a resultant impedance which 1s inversely proportional with frequency. The spacing member not only in sures the proper amount of resistance-in the chamber to damp out resonance, but also introduces a mass reactance caused by the movement of air in the slots which can be pro ortioned to resonate with the stiffness of t e air chamber, formed by the diaphragm and the slot, at a frequency other than the resonant frequency of the diaphragm.

piece and the damping plate whereby the area of the restricted slot remains fixed regardless of atmospheric changes. This arrangement consists in attaching the spacing member to the face of the center pole of the magnet structure so that the plane of the center pole is raised a distance equal to the thickness of the spacing member above the plane of the outer pole of the magnet structure. The damping plate is also attached with the spacing member to the center pole piece which is attached to the center pole of the magnet. The annular pole piece is seated on the ma pole piece and rigidly' xed. Due to the slight difference in the planes of the outer and eticoaxial with the center inner contact edges of the annular pole piece,

the inner edge exerts a constant pressure againstthe spacing member so that the area of the restricted slots formed by the cut-out portions of the spacing member between the outer pole piece and the damping plate is definitely fixed.

In another embodiment of the invention, the damping plate forms the restricted passagewa for the air in the diaphragm chamber an means is provided for varying the area of the restricted path. This arangemerit consists of bi-lateral dam ing members in the'form of two interpose rin s having a plurality of apertures of sma 11 area therein. These rings are attached to the central pole piece and extend across the annular gap between the two pole pieces of the magnet structure so as to completely close constant. This arrangement provides an accurate adjustment of the impedance introduced into the system and provides means whereby this impedance can be individually regulated for each device.

Another feature of the invention relates to providing acoustic chambers on opposite sides of the vibrating diaphragm to equalize the damping effect and improve the quality of the device over a large range of speech and music frequencies. This arrangement consists in forming the restricted resistance paths in the chamber under the diaphragm as previously described and supplementing this arrangement with a hollow chamber on the op osite side of the dia hragm either form in the cap or separate from the cap and inserting a plate or screen having numerous perforations therein in the chamber between the diaphragm and the cap. This feature of the invention may' also be accomplished by providing multiple screens adjacent screens having apertures or slots 0 vary-' ing area, to form successlve paths through which the sound vibrations must pass. This arrangement introduces progresslve filter action on the sound vibrations caused by the vibrating diaphragm, and, due to the vary-' ing values of air resistance of the screens and the capacities of the chambers of the mechanical filter system, the acoustic im ance on one side of the diaphragm supp ements the acoustic conditions on the other side of the diaphragm so that the quality and transmission characteristics of. the device are ap- 1 pedance into the damping chamber.

preciably' improved over the audible frequency range.

These and other features of the invention will be more fully understood from the following detailed descri tion in connectionwith the acompanying rawings. Fig. 1 is a cross-sectional view of an'electrodynamic device made in accordance with this. invention showing the arangement 'of parts for introducing the necessary acoustic ig. 2 is an exploded view in perspective of the magnet "structure showing the detail elements and their relation to each other.

Fig. 3 is an exaggerated outline view of a magnet structure made in accordance with this invention to illustrate the pressure contact of the annular pole piece with the spac member.

Tig'. 4 is a partial cross-section of another embodiment of the invention in which the spacing member is carried by the annular pole plece. I

Fig. 5 is a plan view'showing the configuration of the spacing member shown in F' 4.

Fig. 6 illustrates in cross-section a di erent aspect of'the invention in which the damping plate is provided with staggered holes to form the-restricted paths for the air chamber without employing a separate spacing member.

' the central Fig. 7 shows in pers tive the two part damping plate shown 1n Fig. 6.

Fig. 8 is a plan view of thetwo part damping plate of Figs. 6 and 7 illustrating the adjustable arrangement of the restricted paths extending through the plate.

Fig. 9' is a cross-sectional view of a modified form of the invention illustrating an arrangement for applying damping to opposite n sides of the diaphragm. I

Fig. 10 shows in cross-section a further modification of Fig. 9 in which successive damping members are located on one side of the diaphragm to form a pro essive filter. Fig. 10-A is an enlarged p an view of a quarter section of the damping member v shown in cross-section in Figs. 9 and 10.

the same plane as the top surface of the annular flange 16. The shell or casing is preferabl formed of magnetizable material, such as co alt steel or an alloy thereof, and is of the permanent magnet type so that the central pole and the annular rim form the ends of the magnet structure. The central pole is drilled to receive a bolt18, which secures a dome-shaped circular inner pole piece 19 to outer pole piece 20 is mounted on the shell 15 to form a narrow annular gap between the edge of the inner pole piece 19 and the inner edge of the annular plate pole piece 20. The

annular gap between these pole pieces is obtamed by positioning the annular pole piece on the magnet structure and insert a cylindrical gauge between the edges o f fiie inner and outer dpole pieces. The annular pole piece 20 an the flange 16 aredrilled near the edge thereof at diametrically opposite points and pins 21 are inserted in the flange 16. After the gauge is removed, 'the' annular pole piece ma be removed tocom lete the assembly o the device which wil now de- A thin light diaphragm is attachedto one side of the annular po e piece 20 and comprises a central dome-shaped rtion 22 and an annular flatportion 23. This diaphragm is preferably made of light material, such as aluminum or aluminum alloy, and is mounted on the annular pole piece 204with paper spacing rings between the diaphragm and the annular pole piece 20. The dome-shaped 1e, and a flat annular plate or' iii portion 22 of the diaphra 'operates as a piston and is provided wi 24 attached to the diaphragm at the juncture of the dome-shaped portion 22 and the flat annular portion 23. When the diaphrggm is mounted in the structure as shown in 1g.

1 the coil is coaxially suspended in the annular gap between the edges of the inner pole piece 19 and the annular oi outer poleplece 20 and a thin film of air separates the domeshaped portion from the similarly shaped inner pole piece 19. An annular clamp ng plate is mounted on the annular portion 23 of the diaphragm sothat only a narrow 15 annular area of the diaphra is uncontrolled in its movement when e diaphragm is vibrated.

It is, of course, well known that a metallic dia hragm when vibrated has a natural pe-. ri of vibration well within the speech range, and when speech vibrations are im parted to the air at the natural frequenc of the diaphragm, distortion takes place an the speech is necessarily impaired. This is due to the uncontrolled mechanical motion of the diaphragm at the natural period of vibration. Toovercome this difliculty and to raise the natural riod of a diaphragm such as described yond the ran ofunportance 1 n s eech, the chamber un er the diaphragm is c osed by a damping plate or ring 26, pref erably of brass or ot er non-magnetic material which fits over the cut-out portions of central pole'17 and is attached to the nner pole piece 19 by screws. The damping plate 26 which presents a solid surface across the annular gap between the pole pieces will not allow thepassa e of air mm the chamberunder' the diap ragm and if holes are formed in the damping plate parallel to the annular gap, theseholes or openings w1 ll not offer sufiicientresistance to the 811- n the chamber to substantially raise the period of vibration of the diaphragm.

In accordance with this invention, the air in the chamber under the diaphragm is led out through restricted slots between the annular pole piece 20 and the damping plate 26 so that the air must travel radially at right angles after leavin the annular gap under the diaphragm. ese slots are rov ded by a metallic spacing member or t in shim preferably of nickel or; an alloy metal,

which is-fastened to the inner plole iece19 between the pole piece and t e. amping plate 26. This shim is provided with a plurality of radial extensions 28 which extend across the ap between the'pole pieces and definitely" x the thickness 7 ofthe slots through which the air must pass from the chamber under the diaphragm. The area of the slots introduces a'mass reactance due to; the action of the air forcing its way through.

the slots and this mass can be proportioned to resonate with the stiffness of the air cham an annular coil pieces and the damping her at a difierentfrequency than the ire-1 quency of the diaphragm. -The thin slots between the pole piece and the damping plate also introduce the necessary amount of resistance to the air leaving the diaphragm chamber so that the resonant frequencies of the combined system are dampedout andthe characteristic curve is flattened over a larger range of frequencies The assembly of the damping ringand spacing memberon the central pole piece provides a compact arrangement of the parts and insures a definite space relation between the annular pole piece and the damping plate to. form 1 the narrow slots for the passage of air from the acoustic chamber under the diaphragm,

F i 3 shows an arrangement of a magnet in w ich the spacing member 27 is formed from a solid disc with the extensions 28 projectin across the gap between the pole pieces. The 0 ject of ma 'ng the center portion ofthe spacing member solid is to raise the plane of the central pole a sufiicient distance so that when the annular pole piece 20. is attached tothe magnet structure, the inner edge will bear down on the extensions 28 and insure'a constant spacing of the annular pole piece with respect to the damping plate. This spacing will not be affected by atmos heric changes which may cause the inner go of the outer Eole pieceto be displaced, thereby varying -t e area of the slots between the outer pole piece and the damping plate. Of course, thearrangement shown in Fig.3 is exaggerated to show the relation of the various parts, althou h in actual practice the thickness of, the 's im or spacing member is small and the actual relationship of the parts is hardly noticeable. a Fig. 4 shows another embodiment of the invention in which constant pressure is exerted on the spacing member to maintain a definite area in the slots through which the air must pass from the chamber under-the diaphragm. In this arrangement the inner pole piece 19 isdirectly attached to the central pole of the magnet, and'the spacing. member 29, more clearlyshown in Fig. 5, is attached to the annular pole piece 20 and the inwardly extending ribs 30 brid e the gap between the pole pieces 19 and 20. Ihe damping ring 31 seats on a resilient ring 32, such as rubber, which is located onthe flanged ortion 33 of the central pole of the magnet. 11 this arran ement the resilient ring member exerts su cient pressure on the damping rin and spacing member 29 to maintain the de i-te areas of-the slots between the pole late. While the previously dia'scribed damping, arrangements for the air chamber under the;

idiaphragin utilize a thin shim or spacing member to provide restricted slots for t e passage of air from the diaphragm chamber, the" same efl'ect may be accomplished in. accordll.

ance with this invention as shown in Figs. 6, 7 and 8. p The damping plate is formed of two similar apertured rings 34 and 35, which are supporte by the inner pole iece 19 and are provided with a plurality o circular openmgs 36 which are in alignment in the two rings. One or both rings may be'provided with elongated slots 37 for fastening the rings to the pole piece 19 of the magnet structure. In this arrangement, the values of the mechanical resistance offered by the openings in the two rings may be adjusted to suit individual purposes by misaligning the a rtures 36, as s own in Fig. 8, to form restricted air passages of definite area through which the air from the diaphragm chamber must pass.

' The large bearing surface of the annular pole iece 20 on the two-part damping ring, shown In Fig. 6, prevents any air escaping between these surfaces so that the only path through which the air can pass is through the misali ed openings in the rings 34 and 35.

eferring'again to Fig. 1, after the damping ring 26 and spacing member 27 are attached to the inner pole piece '19 and fastened to the magnet structure by the bolt 18 and the diaphragm and coil are concentrically located in the annular gap of the magnet structure with the clam ing plate securely attached to the ann ar pole piece 20 to cover the greater portion of the periphery of the diaphragm, a perforated cap or car piece 38 is placed on the clamping plate 25 andis formed on its lower surface to rovide a restricted area or chamber above t e diaphragm. The annular pole piece 20 and the related parts associated with magnet 15 are securely clamped thereto by a metallic sleeve member 39 having an inwardly projecting flange 40 which engages'the lower surface of the flange 16 onthe magnet 15. The sleeve 39 is provided with internal threads to engage the threaded portions of the clamping plate 25 and the cap 38 as shown on F' 1.

' The structures heretofore descri d are concerned with the attainment of accurate, reproducible values of mass, stifiness and viscosity for affecting the mechanical motion 7 of the diaphragm on one side thereof toimprove the response characteristics of the device over the audible frequency range in order to obtain a true and natural reproduction of speech. 'lhe same results are obtained with a higher degree in the structures shown in Figs. 9 and 10 except that in these figures the device is disclosed as appl g acoustical resonant systems to both Sid: of the diaphragm in order to increase the range of frequencies reproducible by the device.

As shown in Fig. .9 the clamping plate 41 is positioned above the diaphragm and has a configuration different from'the clampingplate shown in Fig. 1 in that the inner edge is provided with a circular flange or seat 42 closely adjacent to the diaphragm portion 22. A disc or plate 43 having a plurality of elongated thin slots 44 is located on the flange 42 directly above the dome-shaped portion 22 of the diaphragm. The cap 38 seated on the clamping plate 41 has a downwardly projecting portion 45 which securely holds the disc 43 on the seat 42 of the clamping plate 41. A ring or sleeve 39 similar to Fig. 1 securely fastens all the elements to the magnet structure. In this .arrangement the slotted disc 43 above the diaphragm accomplishes the same purpose as the damping plate and restricted slot under the diaphragm and, therefore, supplements the dampin action underneath the diaphragm by contro ling the mechanical motion of the diaphragm in both directions of vibration.' This construction serves to smooth out the response characteristic of the diaphragm over a larger range of speech and music frequencies.

In Fig. 10 the seat 42 on the clamping plate 41 is made deeper so that a plurality of discs such as 43 may belocated in the aperture to form-successive damping members and in effect act as an'a'coustic filter for the air above the diaphragm. In this arrangement a spacing ring 46 is located between two adjacent discs to form an air chamher to affect the stiffness of the filter system.

While in this figure, the slots in the discs are shown in alignment and of the samesize in all the discs, it is, of course, understood that these slots may be staggered in relation or the slots in successive discs may be of different widths to vary the values of resistance of the filter system. Other types of openings in the disc maybe'uscd to accomplish the same purpose. The type of disc shown in Figs. 9 and 10 is shown in enlarged section in Fig. 10- A to illustrate the relative length and width of the slots "aid the relation thereof to the size of the disc. -The slots 44 may be punched in a metallic plate as shown in this figure, but due to the difficulty of securing slots of very thin'width by the punching operation the slotted disc is electroplated to fill up the edges of the slots and thereby produce .slots of suflicient. restricted area which have been found to answer the purpose of this invention.

Fig. 11 shows .a preferred type of a twopiece diaphragm of'relatively light mass in which the dome shaped portion 22 is formed of .9 mils thickness of aluminum and the portion 23 is formed of 2.1 mils of similar material and in which the center portion 22 is fastened at its rim portion 47 to the inner rim of the annular portion 23 by cement, sclder, welding or other suitable means. This type of diaphragm is relatively light in weight and since the dome-shaped portion 22 vibrates as a piston and the greatest part of the annular portion 23 is clamped rigidly between the clamping plate 25 and the'annular pole piece 20 of the acoustic device, it

nulus of the diaphragm vibrates with a free period. However, it has been found that even this small area offers difiiculties in the 3 reproduction of speech and music over the whole frequenc range and therefore the damping and a ditional resonant efiects introduced in the device in accordance with this invention serve to control the mechanical motion of the diaphragm when vibrated. While the device of thisinvention may contain all of the features enumerated, it is understood that the invention is notlimited' to such a combination, since, under certain conditions, the operating characteristics of the device may be sufilciently improved by employing one or more of the various features.

An acoustical impedance diagram showa ing the arrangements of the parts of'the sys-' tem from an acoustic wave transmisslon viewpoint in a form analogous to electrical impedance diagrams of electrical transmission systems is shown in Fi 12. It is well 35 known that the mechanica terms of mass, stifl'ness and friction or viscosity are the equivalent of electrical terms of inductance capacity and resistance,respectivel when considered in the analogous acoustic filter ar- ,rangem ent here to be described. The first section of the diagram represents friction or resistance 50, mass of inductance 51 and stiffuses or capacity 52 in series relation of the diaphragm and coil mountedin the air gap between the poles of a magnet structure such as shown in in vibration by speech currents traversing the movable coil in the air gap will contribute its own period of vibration and, therefore, cause distortion within the range of the speech frequencies due to its natural period.

' In order to overcome this distortion and to increase the range of frequencies to which the device responds, an acoustic system is i .5 added. The second portion of the diagram represent the values of resistance 53, mass 54 and stifiness 55 of this acoustic system and shows the parallel paths that the air under the diaphragm is subjected to by the damp-- ing plate and restricted slot which substantially closes the annular gap in which the coil is located. This introduces a resonance in addition to the resonance of the diaphragm which is difierent in frequency from 1 that of the diaphragm. Sufiicient resistance 7 to damp outthe resonances-of the combined diaphragm and acoustic systems is obtained by use of the damping plate in which the ear must force its way out throughthe restricted slots between the damping plate and the annular pole iece. The third'portion of-the --'-diagram ustrates'sthe sup lemental reso' nant-and damping efi'ect on t eopposite side of the diaphragm as shown in Fig. 9 in w lch resistance 56,;mass 57 and stin'ness 58 are the ig. 1. The diaphragm when set ,annular pole piece on said she values represented. by the chamber above the diaphra and the restricted slots in the disc 43 t ough which the air must ass. In this way the mechanical motion 0 the diaphragm is controlledover substantially the whole speech and music range so that the response characteristic is uniform and substantially free from distortion, thereby improving the rrzproducible response of the dia phragm over e important speech frequency, 75

iii;

'le the features of this invention have been disclosed in various specificstructures 1t is of course understood that various modifications may be made in the details of construction without departing from the scope of the invention as defined in the appended claims.

What is claimed is: I

1. An electrodynamic device comprising a: shell magnet having an inner pole piece, an annular pole piece on said shell forming an annular gap with said inner :pole piece, a d aphragm carried by said an'nular pole p1ece and having a coil lobalted in theannu-r lar gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber under said diaphragm, and a damping ring supported solel by said inner pole piece and substantial y closing the annular gap' in which the coil is located. o

. 2. An electrodynamic device comprising ashell magnet having n inner ole piece, an

annular pole piece on said she forming an annular gap with said inner pole piece, a. d aphra carried 7 b said annular poleiece an having a C011 located in the annuar gap,-said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber under said diaphragm, a dam ing 'ringsup-e ported solely by. said inner po e-piece, anda thin metallic spacing member interposed between one of said pole pieces and said damping ring.

3. An electrodynamic device comprising a a shell magnet having an inner ole piece, an annular pole piece on said she forming an annular gap with said inner-pole piece, a diaphragm carried b said annular pole lar gap, said diaphragm, magnet, pole pieces and gap forming anacousticair chamber under said diaphragm, a damping ring supsaidpole pole piece and damping plate.

'. 4. An electrodynamicdevice comprising a shell magnet having an inn'erpole' piece, an

l forming an annular gap with said inner ole piece,-a

dia hragm carried by'said annu ar pole piece an having a coil located in the annular gap,

said diaphragm, magnet,pole p ieces and gap 1' piece and having afcoi located in the annu- F I forming an acoustic air chamber, a damping ring supported solely by said inner pole piece, an

a spacing member interposed between diaphragm carried by said annular pole piece and having a coil located in the annular gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, a damping plate carried by said inner pole piece and extending across the end of the gap, and a spacing member separating said damping plate from said annular pole piece to provide a restricted gap at right angles to said annular 9 gap.

6. An electrodynamic device comprising a shell magnet having an inner pole piece, an annular pole piece on said shell forming an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and having a coil located in the annular gap, said diaphragm, magnet, pole-pieces and gap forming an acoustic air chamber, a non-magnetic damping member supported by said inner pole piece, and a spacing member having a plurality of radial extensions separating said damping plate'from said annular pole piece. 7. An electrodynamic device comprising a shell magnet having an inner pole piece, an annular pole piece on said shell forming an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and having a coil located in the annular gap, 40 said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, a dampingmember supported by said inner pole piece, and means interposed-between the pole pieces and damping member forming restricted radial openings whereby the mass reactance of the air in the openings is proportioned to resonate withthe stiffness of the air chamber at a frequency other than the resonant frequency of the diaphragm. v

8. An electrodynamic device comprising a magnet having an axialpole and an-outer pole, the faces of said poles being in the same plane, a circular pole piece supported on said axial pole, a ring damping'plate attached to said pole piece, an annular pole piece in contact with said outer pole and concentrically spaced about said circular pole piece,

and spacing means interposed between said. axial pole and circular pole piece to raise the plane of the inner edge'of said annular pole piece, whereby pressure is exerted bysaid annular pole piece on said spacing member.

9. An electrodynamic device comprising a shell magnet having an inner pole piece, an annular pole piece on said shell forming an I annular pole piece 'on said shell forming an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and havlng a coil located in the annular gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, a perfo-' rated ring member bridging the gap between said pole pieces, and means for varying the area of the perforations in said member.

10. An electrodynamic device comprising a shell magnet having an inner pole piece, an annular pole piece on said' shell forming an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and having a coil located in the annular gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, a perforated ring member bridging the gap between said pole-pieces, and adjustable means in face to face relation with said member for varying the area of the perforations therein.

11 An electrodynamic device comprising a shell magnet having an inner pole piece, an annular pole piece on said shell forming an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and having a coil located in the annular gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, and aair of superimposed perforated members bridging the gap between said pole pieces, one of said members being rotatable for misaligning the. perforations in said members.

12. An electrod amic device comprising a shell magnet having an inner pole piece, an annular gap with said inner pole piece, a diaphragm carried by said annular pole piece and havmg a coil located in the annular gap, said diaphragm, magnet, pole pieces and gap forming an acoustic air chamber, a pair of ring members extending across the gap between said 1p-ple pieces having aligned openings theret ough'communicating with the annular gap, one of said members having elon ated slots to permit rotation of said mem r, and means supporting said ring members from said inner pole piece.

13. An electrodynamic device comprising a casing having coaxial pole pieces forming an air-gap therebetween, a diaphragm sup ported in relation to said'pole pieces and aving a coil suspended in the air-gap, anair damping member extending across said air-gap on one side' of said diaphragm, a plate on said 'diaphragm"- having an inner circular flange, and, a plurality of superimposed perforated disc members seated onsaid flange.

14. An electrodynamic device comprising a casing having coaxial pole pieces forming an air-gap therebetween, a diaphragm supported in relation to said pole pieces and having a coil suspended in the air-gap,'an air damping member-extending across said air-gap on one side of said diaphragm, a"

aaid-.ann "er pole-piece and saiddamping eamember." a

plate on said diaphragmhaving an inner' circular flange, and'a plurality of superimposed perfCrated disc members seated on said flange, successive discs having definite 5 diameter perforations.

an air-gap therebetween, a diaphragm supported in relation to said pole pieces and 19. An acoustic device comprising a diaphragm, and means roviding an acoustic network on each sur ace of the diaphragm to control the vibratory characteristics thereof, said networks including acoustic stifiness, mass and resitance elements, said means comprising a member defining a sound passageon one side of the diaphragm, a perforated plate member extending across said passa e, and means spaced from the other side of t a diaphragm to form an air chamber having a substantially annular restricted outlet.

In witness whereof, I hereunto subscribe my name this 10th day of April 1930.

- LEONARD w. GILES.

aving a coil suspended in the air-gap, an air damping member extending across said airgap on one side of said diaphragm, a plate on said diaphra having an inner circular flange, a plurality of superimposed perforated disc members seated on said flange, and a perforated closure member maintaining said disc members in posltion.

a casing having coaxial pole pieces forming an air-gap therebetween, a diaphragm supported in relation to said pole'pieces and aving a coil suspended in the air-gap, an air damping member extending air-gap on one side oi said iaphragm, a plate on said diaphra having an inner circular flange, a plura ity of superimposed perforated disc members seated on said flange, a perforated closure member superacross said impogd on said discs, and a sleeve member clamping said closure member and one of said pole pieces to said casing.

18. An electrodynamic device comprisin flange and a central pole, a dome-shaped pole 17. 'An electrodynamic device comprising a shell magnet having a circumferentia.

piece attached 'to said central pole, an annular pole piece in enga ement with said-circumferential flange, an concentric'with said dome-shaped pole plece to form an annular gap therebetween, a diaphragm having central and outer portions following the contour of said pole ieces, aecoil attached to the periphery 0 said central portion and adapted to vibrate in said annular gap, a

damping member attached to said central pole 1ece and surrounding said central pole,- Jsaid ampmog the member'extend'ing beyon inner'edge saidannglar pole pieeeand a thin metallic spacing member having cutt portions between'said annular pele piece and member, to". maintain a re-v between-the inner edge of 

